This invention relates to a method and apparatus for determining the range of an object from an image of the object or a space containing the object.
There are a number of applications in which it is desirable to determine the range of a an object from an image of the object or a space containing the object. One application is in the area of mobile robot navigation.
In most applications for range determination from images, the range is required to be estimated from a panoramic image. A number of methods exist for capturing panoramic images using conventional cameras. These include rotating a camera about a vertical axis so as to obtain an image or by using multiple cameras looking in different directions. These techniques have a high level of complexity and require a large amount of equipment. International patent application number PCT/AU94/00501 (publication number W095/06303) by the present applicant discloses a panoramic imaging system utilising a single camera and a dome like convex mirror. International patent application PCT/AU97/00815 (publication number WO 98/25245) by the patent applicant discloses an improved panoramic imaging system utilising a single camera and a reflective surface shaped such that the area occluded by the camera is constant or decreases with increasing distance. The disclosure of international application number PCT/AU94/00501 and international patent application number PCT/AU97/00815 are incorporated in this specification by cross reference.
Estimation of range from an image can be achieved in a number of ways many of which involve a comparison of images resulting from a known displacement in some known direction. Various algorithms exist for range computation using two paraxial cameras. This is equivalent to using a single camera to capture images from two different locations and comparing the resulting images. Measurement of range using wide angle imaging has been used in relation to some machine vision. Some systems have utilised a series of motion detectors arranged at different angles to form a panoramic sensor. The detectors are scaled to the amplitude of motion seen at different angles by using detectors tuned to different velocities. This type of arrangement has been used on a mobile robot to provide the depth information to navigate and avoid collisions. A difficulty with this approach is that the motion detectors have narrow fields of view and cannot individually detect complex fields.
Arrangements in which panoramic sensors provide information to an algorithm that tracks the position of features during motion of a mobile robot are known. Using a knowledge of the motion of the features, the range to those regions of image can be determined. This approach suffers from a strong dependence on the feature selection and registration algorithms.
It is an object of this invention to provide an improved method and apparatus for determining range of an object from an image of the object or a space containing the object.
Accordingly, in one aspect this invention provides a method of determining the range of an object including the steps of generating a first image of at least a portion of the object viewed from a first position, producing a second image of said portion viewed from a second position spaced apart from the first position by calculation from said first image using the distance between said first and second positions and assuming all of the portion represented by said first image lies on an arc of constant radius less than the distance to the object, generating a third image of the portion viewed from said second position, and determining the range of the object by comparison of said second and third images.
In a second aspect this invention provides an apparatus for determining the range of an object including means to generate a first image of at least a portion of the object viewed from a first position, calculating means to produce a second image of said portion viewed from a second position spaced apart from the first position by calculation from said first image using the distance between said first and second positions and assuming all of the portion represented by said first image lies on an arc of constant radius less than the distance to the object, means to generate a third image of the portion viewed from said second position, and comparator means to determine the range of the object from said second and third images.
Preferably, the first and third images are generated by moving the effective position of the image generator from the first position to the second position. In an alternative approach two separate image generators of identical configuration can be used at each of the first and second positions.
The first and third images are preferably generated using a panoramic imaging system. More preferably, the panoramic imaging system is of the type described in international patent application number PCT/AU94/00501 or international patent application number PCT/AU97/00815.
Preferably, the images are a circular array representing an image space around a horizon. More preferably, the image is a one dimensional array of pixels that represent intensity in equally spaced regions around a horizon. The regions are preferably slightly overlapping.
In one form of the invention, the determination of range is made from a comparison of the deformation between the second and third images using the relationship that local image deformation is inversely proportional to range.
In another form of the invention, the local optical flow, that is, local angular image translation, is determined from the first and second images and the range determined by triangulation.
As used throughout this specification the term xe2x80x9cimagexe2x80x9d is used to refer to all forms of representation and storage of images by digital or other techniques. Consequently reference to comparison, calculation and other operations with images includes the corresponding manipulation of data indicative of the image whether or not the image is visually displayed at any time.
The invention will now be described, by way of example only, with reference to the accompanying drawings: